09. 约束与索引的联系
之所以把约束和索引放到一起来看，主要是因为主键约束和唯一键约束，它们会自动创建一个对应的索引，先分别看下数据库中的几个约束。 一 约束 在关系型数据库里，通常有5种约束，示例如下： use tempdb s(sid varchar ( 20 ),sname varchar ( 20 ),ssex ssex
之所以把约束和索引放到一起来看，主要是因为主键约束和唯一键约束，它们会自动创建一个对应的索引，，先分别看下数据库中的几个约束。
一 约束
在关系型数据库里，通常有5种约束，示例如下：
use tempdb s ( sid varchar(20), sname varchar(20), ssex ssex) , sage ), sclass varchar(20) unique, constraint PK_s primary key (sid,sclass) ) create table t ( teacher varchar(20) primary key, sid varchar(20) not null, sclass varchar(20) not null, num int, foreign key(sid,sclass) references s(sid,sclass) )
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